We have been investigating nucleic acid structure involved in the control of transcription initiation, termination, and post-transcriptional processing and modification of various RNA transcription products. Using a number of lambdoid transducing phages we have been able to examine the relationship between regulatory function and DNA sequence in several bacterial and viral operon control regions. The characterization of mutants which affect both transcriptional and translational expression in these regions has afforded information on the precise molecular events which lead to control element function. In addition, we have used a variety of defined prokaryotic mRNAs to examine the requirements for translation of these transcripts in a eukaryotic cell-free system. Our results indicate that analogous to many eukaryotic mRNAs, efficient translation of the prokaryotic mRNAs is obtained only upon modification of their 5' termini with a 7-CH3-guanosine "cap" structure. In collaboration, we have also been characterizing a 172 bp DNA segment which is repeated millions of times in the genome of the African green monkey. These sequences are also found recombined into the genomes of certain defective variants of simian virus 40. Several defective virions are presently under study.